1. Field of the Invention
The present invention generally relates to a semiconductor device and a method for manufacturing the same. More particularly, the present invention relates to a semiconductor device in which respective elements formed in the vicinity of a plurality of isolation oxide films of different thickness have stable characteristics, as well as to a method for manufacturing the semiconductor device mentioned above.
2. Description of the Background Art
There has been known a semiconductor device comprising a single substrate on which are provided an array of memory cells and a peripheral circuit made of a plurality of elements. In the former semiconductor device, a memory section including memory cells is provided with an isolation oxide film for electrically isolating the individual memory cells from one another. Further, a peripheral circuit section having the peripheral circuit is provided with an isolation oxide film for isolating the individual elements from one another.
The isolation oxide film in the memory section is required to be formed precisely, whereas the isolation oxide film in the peripheral circuit section is required to have a high withstanding voltage. The precision of the isolation oxide film becomes worse as the thickness of the isolation oxide film increases. In contrast, the withstanding voltage of the isolation oxide film becomes higher as the thickness of the isolation oxide film increases. For these reasons, in order to produce the former semiconductor device having the structure mentioned above, two types of isolation oxide films of different thickness must be formed on a single substrate.
FIGS. 4A to 4F are cross-sectional views for describing a former method for forming two types of isolation oxide films of different thickness on a single substrate.
During the former manufacturing method, an oxide film 12, a nitride film 14, and a resist film 15 are formed on a silicon substrate 10 in this sequence (see FIG. 4A). The surface region of the silicon substrate 10 is divided into a peripheral circuit section 16 and a memory section 17. In the resist film 15, an opening 18 is formed on a predetermined area of the peripheral circuit section 16.
While the resist film 15 is used as a mask, anisotropic etching of the nitride film 14, etching of the oxide film 12, and recessing of the silicon substrate 10 are carried out in that order. After completion of recessing of the silicon substrate 10, the resist film 15 is removed from the surface of the nitride film 14 (see FIG. 4B). As a result of the foregoing processing, a recess 20 is formed in the peripheral circuit section 16 of the silicon substrate 10 in such a way as to correspond to the opening 18 of the resist film 15.
Next, the silicon substrate 10 is subjected to thermal oxidation, whereby an isolation oxide film 22 is formed in the recess 20 (see FIG. 4C). The thermal oxidation of the silicon substrate 10 is carried out under the condition in which a sufficient thickness of the isolation oxide film 22 is ensured; namely, under the condition in which a sufficient withstanding voltage of the peripheral circuit section 16 is ensured.
During the former manufacturing method, a resist film 24 is formed on the nitride film 14 (see FIG. 4D). The resist film 24 has openings 25 formed on predetermined areas of the memory section 17.
While the resist film 24 is used as a mask, anisotropic etching of the nitride film 14, etching of the oxide film 12, and recessing of the silicon substrate 10 are carried out in this sequence. After completion of recessing of the silicon substrate 10, the resist film 24 is removed from the surface of the nitride film 14 (see FIG. 4E). As a result of the foregoing processing, recesses 26 are formed in the memory section 17 of the silicon substrate 10 in such a way as to correspond to the openings 25 of the resist film 24.
The silicon substrate 10 is then subjected to thermal oxidation, whereby an isolation oxide film 28 is formed in the recesses 26 of the memory section 17 (see FIG. 4F). The thermal oxidation in this stage is carried out under a condition for ensuring a required thickness of the isolation oxide film 28. The isolation oxide film 28 of the memory section 17 is not required to have a thickness as thick as that required for the isolation oxide film 22 of the peripheral circuit 16. Thus, the foregoing processing results in the isolation oxide film 28 which is formed in the memory section 17 with a high dimensional accuracy.
As mentioned above, the former manufacturing method enables formation, on the single silicon substrate 10, of two types of isolation oxide films of different thickness, i.e., the isolation oxide film 22 having a high withstanding voltage and the isolation oxide film 28 having a high dimensional accuracy. However, in the former manufacturing method, the two types of isolation oxide films 22 and 28 are formed through different processes. Therefore, the former manufacturing method requires complicated manufacturing processes.
The present invention has been conceived to solve the previously-mentioned problems, and a general object of the present invention is to provide a novel and useful semiconductor device and a method for manufacturing the same.
A more specific object of the present invention is to provide a method for manufacturing a semiconductor device which enables formation, through simple processes, of an isolation oxide film with a high dimensional accuracy and another isolation oxide film ensuring a high withstanding voltage.
The above object of the present invention is achieved by a method for manufacturing a semiconductor device including a plurality of isolation oxide films of different thickness. The method includes a step for forming a nitride film on a silicon substrate. The method also includes a step for forming an opening in the nitride film. The method further includes a step for forming an isolation oxide film below the opening through thermal oxidation. In this method, the opening comprises plurality of openings of different opening diameters and at least one of the opening diameters is set to a value of less than 0.6 xcexcm.
According to the manufacturing method, a silicon substrate is subjected to thermal oxidation while being covered with a silicon nitride film having a plurality of openings. At least one opening diameter of the opening is less than 0.6 xcexcm. In the area where the opening has an opening diameter of less than 0.6 xcexcm, through thermal oxidation an isolation oxide film grows at an rate accurately corresponding to the opening diameter. Therefore, according to the manufacturing method, through single thermal oxidation processing, a plurality of isolation oxide films on whose thickness the opening diameters are accurately reflected can be formed on a single silicon substrate.
A second object of the present invention is to provide a semiconductor device including isolation oxide films of different thickness as well as elements which have stable characteristics while being provided in the vicinity of the respective isolation oxide films.
The above object of the present invention is achieved by a semiconductor device having a plurality of isolation oxide films of different thickness. In the semiconductor device, the peak of isolation impurity concentration is situated at substantially the same depth around all the isolation oxide films. Further, the peak of isolation impurity concentration substantially corresponds to the bottom surface of the thinnest isolation oxide films.
According to the semiconductor device, a peak of isolation impurity concentration substantially corresponds to the bottom of a thinnest isolation oxidation film. The thinner an isolation oxide film, the more likely punch-through is to arise between elements provided across the isolation oxide film. Accordingly, in a semiconductor device having a plurality of isolation oxide films of different thickness, punch-through is most likely to arise below the thinnest isolation oxide film. Punch-through that occurs below the isolation oxide film can be effectively prevented by distributing isolation impurities at high concentration in the vicinity of the bottom surface of the isolation oxide film. Accordingly, the semiconductor device effectively prevents punch-through in the area where punch-through is most likely to occur. Further, according to the semiconductor device, the peak of isolation impurity concentration is situated at substantially the same depth around all the isolation oxide films. The foregoing impurity concentration can be achieved without changing the condition for implanting isolation impurities. Thus, according to the structure, a superior punch-through resistance can be ensured over the entire surface of the substrate, and superior productivity of a semiconductor device can also be ensured.
A third object of the present invention is to provide a method suitable for manufacturing a semiconductor device including isolation oxide films of different thickness as well as elements which have stable characteristics while being provided in the vicinity of the respective isolation oxide films.
The above objects of the present invention are achieved by a method for manufacturing a semiconductor device including a plurality of isolation oxide films of different thickness. The method includes a step for forming the plurality of isolation oxide films of different thickness on a silicon substrate to. The method also includes a step for implanting isolation impurities into regions below the plurality of isolation oxide films of the silicon substrate, under the same conditions. In the method, an implanting condition of the isolation impurity is set such that the position of the concentration peak of the isolation impurities in a depthwise direction substantially corresponds to the bottom surface of the thinnest isolation oxide film.
According to the manufacturing method, isolation impurities are implanted into the entire surface of the silicon substrate under identical conditions such that the peak of impurity concentration substantially corresponds to the bottom surface of the thinnest isolation oxide film. According to the manufacturing method, therefore, a semiconductor device having superior punch-through resistance over the entire surface thereof can be manufactured in simple processes.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.